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Full text of "Handbook Of Chemical Engineering - I"

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PYROMETRY
439
leads. Insert this in the well, connect it to a portable indicator and measure the secondary cold-junction temperature at the indicator with a thermometer. With a well 10 ft. or more deep and properly located, it is necessary to measure the temperature possibly only once a month. Another method is to lower a thermometer into the well. The thermometer is wrapped with a few layers of cloth leaving the stem exposed near the expected reading. It is left in the well for 30 min. and read quickly after raising. The recorder or indicator is adjusted
FIG.  15.—Simple junction box installation.
to correct for this temperature or the records may be marked "cold junction" arid corrections applied later.
All buried leads to the cold junction should be water-proof insulated, and the junction well should be made watertight. The compensating leads, particularly those for base-metal couples, will generate a large voltaic electromotive force if they are wet and are not insulated with water-proof covering.
Depth of Immersion of Couples.—Thermocouples immersed in furnaces of the various industrial types must be carefully protected. Heavy iron tubes and frequently larger auxiliary protecting tubes of various materials are employed. The thermocouple calibrated in the laboratory will give the temperature of the hot junction when installed in the protecting tubes but there is no certainty that this temperature is that of the furnace. Conduction along the protecting tubes may be so great that the temperature of the couple is far below that of the furnace. Conduction losses affecting the temperature of the hot junction may be reduced and even eliminated by using a sufficient depth of immersion but it is not always possible to do this and it is difficult to determine when the depth of immersion is sufficient. For example, an immersion of 5 cm. might be satisfactory for one type of furnace or for a molten bath, and 30 cm. insufficient for a certain muffle furnace. Each particular case requires special consideration.
The general practice is to use as deep immersion as is conveniently possible and trust that this is satisfactory. Two methods may be suggested for investigating the question, but neither one is conclusive. First, remove the couple alone from the fixed installation, leaving all protecting tubes in place. If it is impossible to remove the iron tube from the couple use a similar tube in the fixed installation without the couple. Explore the temperature inside the protecting tube with an unprotected